Information is written onto a magnetic medium by magnetizing elements within the medium. These magnetized elements produce a magnetic field which can be detected and converted to an electrical signal by a read head as the magnetic media passes by the read head. A common type of read head for carrying out this conversion is the magnetoresistive (MR) read head.
A simple MR head consists of a thin film of magnetoresistive material, such as Permalloy, between two insulating layers. When the MR layer is formed, a magnetic field is typically applied in a direction parallel to the plane of the thin layer. Thus, the MR layer exhibits a uniaxial anisotropy with an easy-axis of magnetization parallel to the direction of the applied field. If an external magnetic field, such as from the magnetic medium, is applied normal to the easy-axis, the magnetization direction of the MR layer will rotate away from the easy-axis and towards the direction of the applied magnetic field. This magnetization rotation causes a change in resistance in the MR layer. When no external field is applied, the resistance is greatest. The resistance decreases with increasing applied field. For practical geometries of the MR layer, resistance as a function of applied field traces a bell-shaped curve. The MR head is often biased with an applied current such that a zero magnitude applied field results in a resistance near an inflection point on the resistance curve. Thus, small changes about a zero magnitude applied external field result in nearly linear changes in resistance.
To accommodate increasing densities of data stored on magnetic media, the geometries of read heads continue to shrink. One difficulty encountered is the increasing effect of Barkhausen noise. As the width of the MR layer is narrowed, the MR layer tends to split into magnetic domains, resulting in demagnetization. In the presence of an increasing externally applied field, the domain walls can make sudden movements, causing jumps in the output signal. Two methods exist to reduce or eliminate Barkhausen noise. The first is to increase the effective length of the MR layer. Lengthening the MR layer reduces the effect of demagnetization at the ends and, hence, results in a greater retention of a single magnetic domain. The main difficulty with this technique is that the resulting increase in read head size is contrary to the need for increased data density on magnetic tapes. The second technique uses a small magnetic field in the direction of the easy-axis to induce a single domain state in the MR layer. An implementation of this method uses permanent magnets placed over the ends of the MR layer. These magnets strongly pin the domains of the MR layer under the magnets and create a weak longitudinal magnetic field in the MR layer between the covered ends. Difficulties with this implementation include complex geometries and additional processing steps required to implement the additional permanent magnetic.
In addition to Barkhausen noise, cross-sensitivities to other parameters, such as temperature asperity noise, feed through noise, drive noise, and the like, can affect the performance of the MR head. A dual active element MR read head minimizes cross-sensitivities. The dual active element MR head includes two MR layers in parallel separated by an insulating layer. Two additional insulating layers, one on each end of the structure, insulate the MR layers from surrounding materials. The two MR layers are connected in parallel to a source current such that current flows in the same direction through both layers. The fringe field produced by the current flowing through each MR layer biases the adjacent layer. Hence, an externally applied magnetic field produces an increase in resistance of one MR layer and a corresponding decrease in resistance of the other MR layer. A differential amplifier with an input connected to each MR layer converts these changes in resistance to an output voltage. Environmental changes to both MR layers, such as changes in temperature, appear as common mode inputs to the differential amplifier and, hence, are rejected.
The current is supplied to each MR layer through conductors. The conductors are typically constructed from metal such as gold or copper to reduce lead resistance and, hence, increase signal amplitude. Low conductivity metals are susceptible to one or more of corrosion, wear, and abrasion. This is particularly true when the magnetic media is tape, which comes into contact with and abrades the read head surface. Therefore, the conductors are typically connected at the side of the MR layer opposite from the exposed surface of the read head. One problem created by conductors connected to the MR layer back edge is the creation of two right angle bends in the current path through the MR layer. The resulting current path has a distinctly nonuniform current density near regions where current enters and exits the MR layer. Also, the right angle bends in the current path result in areas of low current density in the corners opposite where the conductors join the MR layer in the rectangularly shaped MR layers. These low current density regions generate very low fields and, hence, are more susceptible to domain formation and resulting Barkhausen noise.
What is needed is a dual active element MR read head with reduced Barkhausen noise susceptibility. The read head should have MR layers with a more uniform biasing than present designs. The read head should have a simple construction which is inexpensive to manufacture and which is compatible with existing thin film designs.